The present invention relates to apparatuses for solid/liquid separation carried out by filtration on membrane or textile materials. More particularly, the invention relates to improvements made to filtration apparatuses of the above type, intended more specifically for the treatment of suspensions of sludge resulting from physico-chemical or biological purification operations (activated sludge, mixed liquors). It applies especially to the purification of wastewater by biological means.
It is known that the biological purification of wastewater relies on the culturing of purifying bacteria in specific systems. The system most commonly used is activated sludge, that is to say a culture of these bacteria in suspension in treated water, usually called the mixed liquor. New systems, called mixed cultures, make use of this sludge, mixing it with a suspended medium material.
The effectiveness and the reliability of these systems rely on separation between this activated sludge and the treated water, this separation constituting clarification. At the present time, the clarification is in almost all cases achieved by settling, but in a few special cases by filtration.
The essential characteristics of these two known clarification techniques will be recalled below:
Secondary settling tanks or clarifiers are plants designed to separate the mixed liquor from the treated water by simple settling. The use of these clarifiers in wastewater treatment stations has two major limitations: firstly, the sludge concentration in the biological basin is limited to six grammes of dry matter per liter of mixed liquor, which translates to large plant volumes, and, secondly, it is necessary, if a nitrification step is carried out, to denitrify the effluent so as to prevent flotation of the sludge in the plant and consequently the leakage of suspended matter into the treated water.
Moreover, most existing stations are confronted with many problems which degrade the settlability of the sludge, especially the proliferation of filamentary bacteria causing expansion and foaming problems, variations in pH or in salinity, and influx of toxic products, which often result in the settling operation being seriously disturbed.
In short, malfunctions of clarifiers are a frequent occurrence and often result in dramatic leakage of sludge into the subsequent treatment steps or into the natural environment: this lack of reliability is therefore incompatible with the latest discharge standards.
In clarification by filtration, the filtration is carried out by ultrafiltration membranes (having pore sizes of generally between 0.001 and 0.1 xcexcm) or microfiltration membranes (having a pore size of generally between 0.05 and 5 xcexcm). These separation processes, when applied to the field of biological sludge filtration, are generally known by the name MBR (membrane bioreactors). In all these systems, energy is applied to the sludge, at the surface of the membrane, so as to limit membrane fouling and to increase the flow rates and/or the duration of the filtration.
A first type of MBR is characterized by the use of a membrane located outside the biological basin for performing the separation, thereby making it possible to apply a large amount of useful energy to the surface of the membrane. In this case, the sludge is made to flow at a high rate through the filter, tangentially to the membrane, so as to limit membrane fouling. The transfer pressure (TMP: transmembrane pressure) is between 0.5xc3x97105 Pa and 5xc3x97105 Pa. In this way, filtration outputs of about 50 to 200 1/h per m2 of membrane are obtained. These outputs are maintained for periods of about 1 to 2 weeks before the membrane becomes clogged up, the latter then being chemically regenerated. The main limitation of these MBRs stems from the energy consumption associated with making the sludge flow through the filter. The high flow rates required entail in fact an electrical consumption of about 1 to 5 kWh per m3 of water treated. For this reason, the field of application of these MBRs is limited to small-capacity plants and more particularly for highly contaminated effluents.
A second type of MBR is characterized by the fact that the filtration of the sludge is performed on membranes located within the biological basin. In this case, the filter is immersed directly in the biological basin, turbulence being created by aeration and/or stirring within the sludge to be filtered. The treated water is collected under gravity or by suction pumping through the membrane, which translates to transfer pressures of generally between 0.1xc3x97105 Pa and 105 Pa. The level of energy applied to the surface of the membrane is lower than in the case of the first type of MBR described above, thereby making it possible to reduce the energy consumption. However, the outputs obtained are no more than about 5 to 50 liters per hour per m2 of membrane. These low outputs therefore result in the use of large filter areas, the cost of which very greatly penalizes the process, especially for large-capacity plants and more particularly when the effluents are not highly concentrated.
Moreover, there are other filtration processes which combine high-output performance with low energy consumption but, to various degrees, these known processes prove to be unsuitable for the filtration of concentrated biological suspensions. These processes include frontal-filtration processes in which the filtration is carried out on a medium with coarse pores covered with a preformed prelayer (addition of a diatomaceous suspension, etc.) providing the actual separation (cartridge filters, etc.), such systems operating in a discontinuous manner, by xe2x80x9cfiltration/unclogging/prelayer formationxe2x80x9d cycles. The filtration periods are characterized by a greater increase in the transfer pressure due to the build-up of matter on the surface of the prelayer and to the migration of fine particles into this layer. This pressure structures the deposition on the surface of the coarse filtration medium. When the pressure reaches a critical value of about 105 Pa to 106 Pa, the xe2x80x9cprelayer+trapped particlesxe2x80x9d combination rendered coherent by the pressure is removed by a very vigorous washing operation using countercurrent water or air. Unfortunately, these processes are difficult to apply to the filtration of biological suspensions since these particles are usually highly compressible: when the suspensions are concentrated, the rapid growth of the deposit of particles on the surface of the filtration medium results in a dramatic increase in the hydraulic resistance, which in turn results in cessation of filtration.
In conclusion, the MBR filtration techniques constitute, for low-capacity plants, a satisfactory technical alternative to conventional settling-based clarification processes; on the other hand, their construction and/or running costs remain higher than those of the said conventional processes. For large capacities, greater than a few thousands of m3 per day, there is at the present time no satisfactory solution. The present invention is specifically aimed at providing a solution to this problem.
Consequently, the subject of the present invention is an apparatus for the separation of a solid/liquid suspension by filtration, especially of sludge from a biological or physico-chemical purification liquor, on a textile or membrane filter material, characterized in that it consists of at least one plate immersed in the suspension to be treated, the said plate being inclined with respect to the vertical and made in the form of a cavitied plate, comprising one or more compartments, only the lower face of which is covered with the filter medium, the upper face being solid, the internal volumes of the said plate being connected to a manifold or to a duct which gathers the treated water, and gas-flushing means are provided, these being positioned at a lower level than the filter medium so that the gas bubbles emitted by the said flushing means flow away along the filter medium.
According to one embodiment of this invention, the separation apparatus consists of several cavitied plates having one or more compartments, the said plates preferably being arranged parallel to each other. They may be equidistant from each other or separated from each other with a variable spacing.
According to the present invention, the treated water may be recovered by the said manifold or the said duct either at the top or side of the said plate or plates, or at the bottom of the latter, the latter method of operation preventing or at least limiting any deposit build-up.
According to a preferred embodiment of this separation apparatus, the said plate or plates is or are inclined at approximately 5 to 60 degrees with respect to the vertical. In the illustrative embodiment in which several plates are provided, the spacing of the latter is preferably between approximately 0.5 and 20 cm.
According to the present invention, the filter medium is preferably a coarse medium made of a membrane or textile material having a pore size of between 0.2 and 100 xcexcm.
According to the present invention, the gas-flushing means may be produced in the form of an array of air diffusers placed beneath the filter membranes. These diffusers may be placed at the bottom of the biological basin or tank receiving the apparatus, and their orifices emerge under the filtration plate or plates or in the lower part of the said tank.
According to another embodiment of the invention, the gas-flushing means are incorporated into the said plate or plates, these means being made in the form of separate chambers which extend the lower part of each plate and are fed with gas via a feed pipe, each of the said chambers being provided with at least one discharge opening which is positioned so that the gas bubbles flow away along the filter medium of each plate.
According to the invention, the gas may be air, oxygen, ozonized gas or an inert gas and its flow rate is preferably about 0.05 to 10 m3/h per linear meter of plate width.
According to the invention, the gas is injected continuously, discontinuously, periodically or randomly, at a constant or variable flow rate.
Further features and advantages of the present invention will emerge from the description given below with reference to the appended drawing which depicts two illustrative embodiments, these being devoid of any limiting character. In the drawing: